Scientists believe that methane could have been left by micro-organisms that either became extinct millions of years ago, and left the gas frozen below the planet’s surface, or that they still exist in some form.

Is NASA involved?

NASA has a similar mission, called MAVEN, which is concentrating on monitoring ‘atmosphere escape’ from Mars.

ESA says both missions are very different but that they will compare results.

What’s next?

TGO is expected to begin its monitoring of Mars in 2018, before the launch of the rover onto the planet’s surface two years later.

Longer term, Elon Musk’s SpaceX is developing a massive rocket and capsule to transport large numbers of people and cargo to Mars with the ultimate goal of colonising the planet. Musk hopes to launch the first crew as early as 2024.

Main picture: Mars as seen by the webcam on ESA’s Mars Express orbiter on 16 October 2016, courtesy of the ESA